European patent EP-B-0 599 672 describes a high frequency signal transmission cable comprising at least one conductor unit. Each unit has at least two sets of conductors, such as a quad of twisted conductors, each conductor being insulated by a first insulating sheath. The unit, or a plurality of said units, are surrounded by a second sheath constituted by a thin extruded covering of insulating material situated in contact with the conductors or the set of conductors without said conductors being embedded in the sheath constituted by the covering. The insulating material simultaneously provides sufficient stiffness to hold said conductors in a determined position while said cable is being handled, and sufficient elasticity to make such handling possible.
Unfortunately, to obtain a level of cross-talk that is compatible with good transmission in a cable comprising at least one unit of at least four twisted conductors in the form of sets of conductors that are pairs or quads, the twist pitches of said sets of conductors must be different. The problem for data transmission is that given that each set of conductors has a different electrical length, signals applied simultaneously at one end of the cable to each of said sets of conductors reach the other end of the cable with relative time shifts. The problem becomes particularly troublesome when data transmission rates are high.
At present, this difference is compensated by active equipments associated with passive buffer memories or "buffers". These memories are expensive, and the storage capacity they require is becoming ever greater.
Thus, the problem which arises is that of mitigating the transmission time differences between conductors in a given set of conductors, and the conductors in another set of conductors, said sets of conductors being situated in a unit of a data transmission cable. The object of the present invention is to make a data transmission cable which solves the problem posed in that it enables propagation time differences between the conductors of said sets of conductors to be reduced considerably, i.e. it makes it possible in general to obtain such propagation time differences that are less than two nanoseconds per hundred meter length of cable (2 ns/100 m).